Hydraulic oil is different to regular lubricating oils. Yes, it does lubricate, and many hydraulic oils like Mobil DTE Light can be applied in both hydraulic and non-hydraulic systems, but what makes it special is that it also acts as the power transfer medium, so it plays two roles in a hydraulic system.
Like any other lubricant, it needs a range of properties to be effective and protect equipment, such as anti-wear additives, hydrolytic stability, demulsibility, and protection against corrosion and rust, to name but a few.
In addition, to be efficient as power transfer medium, a hydraulic oil needs to a high bulk modus, which means it doesn’t easily compress under pressure. It also needs a high viscosity index, so its viscosity won’t change much at high temperatures. Any changes in either of these properties can reduce or improve the efficiency of a hydraulic system, meaning more or less energy is outputted as work to get things done.
Of course, it stands to reason that a completely uncompressible fluid would be the most efficient for hydraulic systems, preferably with an unvarying viscosity of 25 centistokes. Unfortunately this doesn’t exist, so the challenge for lubricant manufacturers is to get as close to this as possible with oils like Mobil DTE Light and the Mobil DTE 10 Excel range.
The bulk modus conundrum
Improving the bulk modus represents a problem, because it is not something that can be improved with additives. It’s basically an inherent property of the base oil, so all lubricant manufacturers can do is choose the most suitable base oils. Improving the viscosity index (VI), however, is a different matter. Using synthetic basestocks can lead to a higher VI, for example. There are also polymer additives known as VI improvers that can be added to an oil to raise its VI. While these have been around for more than half a century, they were typically not shear stable in hydraulic applications.
Fortunately, technology has moved on, and VI improvers can now achieve VI values in excess of 150. This is evident in the Mobil DTE numbered range. Taking, as an example Mobil DTE 26, which has an ISO viscosity of 68, Mobil lists its VI as 98. This is by no means bad, but then look at its more advanced equivalent, Mobil DTE Excel 68, which also has an ISO viscosity of 68. You will see Mobil has improved its VI to 156.
So, what does this mean in practice?
Mobil has performed tests under laboratory conditions for standard hydraulic applications, both with oils from the Mobil DTE 20 series and the Mobil DTE 10 Excel series. It found the Excel series could achieve up to a 6% improvement in hydraulic output. Depending on the application, this could mean greater output (better productivity) or reduced energy consumption (reduced operational costs). They also found the newer oils to have a much longer life than their older counterparts, sometimes as much as three times longer. This can bring the benefits of reduced oil requirements and lower maintenance costs, as well as less scheduled downtime for servicing.